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Two balance experiments were carried out using lactating ruminants. In Experiment (Exp.) 1, seven goats per treatment were fed a constant roughage ration plus one of two concentrates, differing in their crude protein concentration. In Exp. 2, which followed a switch-over design, four dairy cows were fed a complete low-protein ration either with or without urea supplementation. According to official recommendations, N intake was either adequate or reduced by about 27 and 18% for goats and cows, respectively. Following a single intravenous dose of 15N-labelled urea, milk, faeces and urine were collected for periods of two weeks in Exp. 1 and of 10 days in Exp. 2. Lowering N intake caused reductions of milk urea concentration in both species but did not significantly affect intake, milk yield or milk protein concentration. In both experiments, lowering of N intake caused substantial reductions in excretion of urea in urine and milk but did not affect excretion of faecal N or urinary non-urea-N. N balances di...

P homeostasis affected by high or low Ca and/or P supply in preruminant goats was characterized by balance studies in vivo. The main excretion pathway was the renal P(i) excretion whose extent was modulated by variations in dietary P and/or Ca supply. Faecal P excretion remained low irrespective of dietary regimen. The balance data were combined with respective in vitro data on P(i) transport properties and their adaptation in response to changes in dietary Ca and/or P intake. Therefore, P(i) transport capacities were determined by P(i) uptake into brush border membrane vesicles of jejunum and kidney. Epithelial P(i) transporters were determined semiquantitatively by northern and western blot analyses in jejunum, kidney and salivary gland. Renal P(i) transport was downregulated by doubling dietary P supply while doubling both, Ca and P as well as restrictive Ca at unchanged P led to slight, but not significant reductions in renal P(i) transport. Jejunal P(i) transport was reduced by P excess (doubling P and doubling both, Ca and P), but only NaPi IIb protein expression was significantly diminished. In conclusion, the significance of epithelial adaptation to dietary Ca and P supply for P homeostasis is discussed in preruminant goats.